1. Introduction

There are many bridges in the world, but a number of them actually become destroyed because of natural causes. Here are the disasters that caused the fall of bridges, and their examples.

Earthquakes

Most bridges are non earthquake-proof. Most of them collapsed whenever an earthquake occurs.

Two examples happened on the 17th October 1989, the Loma Prieta Earthquake hit the Bay Area in San Francisco with a Richter scale of 6.9 and 7.1 for the moment magnitude and surface wave magnitude respectively. It only lasted for 15 to 20 seconds, however, 62 people died and an estimated average of 3757 people were injured. Not only this, but the earthquake left a median of 7500 people homeless due to damage to houses and buildings in the Area.

Not only that, but the Oakland Bay Bridge suffered from the earthquake – the upper part of the eastern truss collapsed, causing it to block the lower deck. Unfortunately, this caused an unlucky driver to drive off the bridge, resulting in his death.

As a result of the damage, the bridge was closed for a month to repair the damage and make changes to the structure to make safety precautions of commuters. However, the bridge is extremely popular as it allows city workers to go to and fro from the suburbs into San Francisco. (https://extendedprojectelodie.wordpress.com/2012/12/30/case-study-3-collapse-due-to-earthquakes-2/, 2012)

Fire (and Boat Impact)

Some bridges collapse due to fire. Fire is a rare cause for modern bridges collapsing, or making them severely damaged but it still happens occasionally. Most of them often catch fire, due to tanker trucks carrying large amounts of highly flammable goods crashing into the bridge. Boats also often have a lot of mass. Therefore when they crash into bridge pilings or piers it is enough to knock down a bridge. If the bridge still can take the impact, the fire often melts the steel making the melted steel that is malleable not being able to hold the load that they fall.

An example of the boat impacting a bridge is the Tasman Bridge disaster that occurred on 5 January 1975, in Hobart, Tasmania, when a bulk ore carrier travelling up the Derwent River collided with several pylons of the Tasman Bridge. It caused a large section of the bridge deck to collapse onto the ship and into the river below. Twelve people died, including seven crew on board the ship and the five people who fell 45 m after driving off the bridge.

Another example of a fire collapsing a bridge is a massive bridge over Interstate 15 in the Mojave Desert that links los angeles and las vegas that caught on fire after a metal-cutting accident set it ablaze, blocking a major route between Southern California and Nevada. The blaze erupted Monday afternoon and it burned through the night.( Press Association, 2014)

Floods

Floods cause bridge collapses in a few ways. Severe floods can cause rivers and creeks to overflow, picking up trees, cars and parts of houses. When the river goes under a bridge, the high water level smashes the debris into the bridge. If the impact doesn't destroy the bridge immediately, the weight of the piled up combined with the force of the flowing water pushing on it can bring the bridge down.

Flooding can collapse bridges in the long run -- by gradually wearing away the earth around and underneath the bridge piers. This process is known to bridge engineers as scour, and occurs whenever bridge foundations are placed underwater.

The natural flowing of the water can produce scour over many years, but bridges are built to withstand that type of sedimentation. Engineering techniques such as laying layers of heavy rocks, can prevent scour. However, floods dramatically increase the force and volume of water affecting the bridge, and the damage to sediments can cause a bridge to collapse immediately or even months later. A study by the American Society of Civil Engineers determined that 53 percent of all bridge collapses are caused by flood and scour.

An example of this is the ancient bridge in North Yorkshire which partially collapsed into the swollen River Wharfe, during a flood. Large sections of the 18th century structure was slipping into the flood. The bridge had already been shut due to structural concerns caused by flooding caused by storm. The collapse of the bridge caused a gas leak in the area as broken pipes were exposed. (Telegraph Video, 2015)

Design Flaws

There are many bridges in the world, but a number of them actually become destroyed because design flaws, here are the reasons why.

Based on science.howstuffworks.com, amongst one of the top ten reasons as to why bridges collapse, is due to design defects, and that the bridge’s design was already doomed to fail the moment it was drawn on the blueprint.(GRABIANOWSKI, E. (n.d.) 2011.)

Based on the book “The Science of a bridge collapsed” it stated that according to The Federal Highway Administration, it reported that there were 603, 310 bridges in the United States in 2009, and amongst them 26.5% of them did not meet the federal maintenance standards. Suggesting that they need some kind of monitoring or repair. (Brooks, B. N. 2014)

An example of how deadly could a bridge disaster get, was in 1907, August 29, where the Quebec Bridge collapsed due a design defect which robbed 75 lives and leaving 11 injured. The bridge was expected to be a 67 feet wide multi-functional bridge, where it could accommodate 2 railway tracks, two streetcar tracks and 2 roadways. The bridge design then for the Quebec bridge was a cantilever bridge, and was the largest one ever built.

After a few meetings, P.L. Szlapaka’s of the Phoenix Bridge Company design had been approved. The wor-

king drawings was drawn in 1905. However the working drawings took over seven months before it reached Mr Theodore Cooper.(a renowned bridge builder from new york, who came to oversee the building process) By then, construction work had already taken place. While viewing the working drawings, he realised that the estimated weight of the span was off. Cooper then decided to allow construction to continue, stating that 8 million pounds was within the engineering tolerance. That decision had caused one of the world’s greatest bridge collapse in history.

During the few years that led up to the day where the bridge collapsed, they found out that there were two girders that had been bending over time and were getting out of alignment. Despite the warning, the inspection team thought that it was not that serious, and didn’t cared much in terms of the design, causing the Quebec bridge to collapsed.

One other example of a bridge disaster is the Tay Bridge Disaster. This bridge disaster is one of the most famous bridge disasters, and it is still to date one of the worst structural engineering failures in the British Isles. The bridge was designed by Mr Thomas Bouch, and he was responsible for the construction and maintenance of the bridge too. Most of his bridges were lattice girders supported on slender cast iron columns braced with wrought iron struts and ties. The first Tay Rail Bridge was opened in February 1878, it measured up to nearly two miles long, consisting of 85 spans. At that time, The Tay Bridge was known as the longest bridge in the world. It’s spans carried a single rail track; 72 of which were supported on spanning girders below the level of the track, while the remaining 13 navigation spans were spanning girders above the level of the track. Up till this day, there has been many answers as to the fundamental cause of the collapse of the Tay Bridge.

According to the court of enquiry on the collapse of the Tay Bridge disaster, it was pointed out that the fall of the bridge was occasioned by the insufficiency of the cross bracing and its fastenings to sustain the force of the gale. It was pointed out that if the piers, and in particular the wind bracing, had been properly constructed and maintained, the bridge could have withstood the storm that night that took place on the 28 December 1879.

From the above examples, we could see how fatal it is to have a poor bridge design. There are a total of 7 different bridge designs based on their structure. They are the Arch, Beam, Cable-stayed, Truss, Cantilever, Tied arch and suspension bridge. (Types of Bridges. (n.d.). Retrieved January 12, 2016,)

The first design is the Arch bridge. The Arch bridges can only be used where the ground or foundation is solid and stable because unlike girder and truss bridges, both ends of an arch are fixed in the horizontal direction.These bridges have arch structure under the main bridge as a main structural component. They are made with one or more hinges, the more the hinges, the stronger it is. The arch bridge is one of the most naturally strong bridge design. The arches properly dissipate the force applied on the bridge, taking away much of the tension coming from underneath the structure. (Types of Bridges. (n.d.). Retrieved January 12, 2016,)

The next design is known as the beam bridge. These bridges are supported by several beams of different shapes and sizes. They can be inclined or V shaped.The beam bridge is the simplest and most common type of bridge construction. A horizontal plank (beam) runs across the gap and is supported by columns (piers) underneath. The farther apart the spaces between piers the weaker the bridge is, which is why most beam bridges are less than 250 meters in length.(Types of Bridges. (n.d.). Retrieved January 12, 2016,)

Next we have the Cable stayed bridges are continuous girder bridges but with a tower or two erected in the middle. Cables stretch down from the towers diagonally to support the bridge. eg. golden gate bridge. Bridge that uses deck cables that are directly connected to one or more vertical columns. Cables are usually connected to columns in a harp design or a fan design. The cable-stayed bridge is very similar to the suspension bridge in terms of design in such a way that the deck is supported by cables. The only difference between these two types of bridges is that in the cable-stayed bridge only one tower is needed.(Types of Bridges. (n.d.). Retrieved January 12, 2016,)

The truss Bridge. All beams in the bridge are straight. Comprised of many small beams that can together support large amount of weight. It takes a hollow skeletal structure.Among the modern bridges, the truss bridge is among those with the oldest designs. It is has triangular units along the span of the bridge. Some designs have these triangular units above the beam (through truss), while other have it below the beam (deck truss).It is a very popular bridge design that uses diagonal meshes of posts above the bridge. The two most common designs are the king post, which is two diagonal posts supported by single vertical post in the center) and queen posts, which is two diagonal posts, two vertical posts and a horizontal post that connect two vertical posts at the top.(Types of Bridges. (n.d.). Retrieved January 12, 2016,)

The Cantilever bridge, these bridges are similar in appearance to arch bridges, but they support their load not trough vertical bracing but trough diagonal support. They often use truss formations both below and above the bridge.(Types of Bridges. (n.d.). Retrieved January 12, 2016,)

The tied-arch bridge. They are similar to arch bridges, but they transfer weight of the bridge and load to the top chord that is connected to the bottom chords in bridge foundation.(Types of Bridges. (n.d.). Retrieved January)

Finally we have the Suspension Bridge. They are similar to arch bridges, but they transfer weight of the bridge and load to the top chord that is connected to the bottom chords in bridge foundation.(Types of Bridges. (n.d.). Retrieved January 12, 2016,)

We would be using the west point bridge designer 2nd edition to build the bridge. The WPBD is a sophisticated program and may seem complicated to the first-time user. It comprises of an animation simulation, a building board, and reports that show essential information such as design cost, weight of each member, the different intensities of compression and tension, etc. There is a challenge in the program, which is to build the most stable bridge with the least cost. This includes factors such as material, size of each singular members, etc. Users are able to load the testing animation once a change has been made to the design and when the truss bridge designed is properly made with the truss format. The animation allows users to see the compression and tension in the bridge, and allows them to easily spot errors in their design. A large animated truck exerts a force of 225 kilo-Newton (kN) per lane; 450 kN total load which meets the American Association of State Highway and Transportation Official H25 Standard. A table containing both compression and tension forces experienced by each member is easily calculated, and strength capacity of each member. All bridge members have something called the modulus of elasticity which is the amount of flex or bending force and object can hold while still being able to possess the ability to return to its original shape.

(MITTS, C. R. 2013)

(b) Independent variable

The Independent variable is the design of the bridge. This is the design that we used.

(c) Dependent variable

The dependent variable is the type of load and the mass of the bridge.

(d) Controlled variables

(a) Ice cream sticks are used to build all the bridge

(b) Dimension of all the bridges shall be 20cm in height, 12cm in width and 60cm in length

(c) The load machine would be used as a load throughout all the bridges

(d) The location Physics Lab.

(e) The length of the bridge will be 60cm

1.1 Research Questions

1.2 Hypothesis

We feel that the heavier the bridge, the weaker the bridge, as the bridge is already very heavy, and if we were to add on more load, it would be too heavy for the bridge, therefore collapsing, as the bridge itself is already very heavy, adding more mass to it, would mean that the bridge would have a higher chance of collapsing.